nix3-eval

new Nix 3

TLDR

Evaluate Nix expression

$ nix eval --expr '1 + 2'

Evaluate flake attribute

$ nix eval [.#packages.x86_64-linux.default]

Evaluate nixpkgs attribute

$ nix eval nixpkgs#hello.version

Output as JSON

$ nix eval --json nixpkgs#hello.meta

Raw string output

$ nix eval --raw nixpkgs#hello.name

Read from file

$ nix eval -f [file.nix]

DESCRIPTION

nix eval is the new Nix 3.x command for evaluating Nix expressions. It replaces nix-instantiate --eval with a cleaner interface and flake support.

PARAMETERS

--expr expr

Evaluate expression.

--json

Output as JSON.

--raw

Raw output (no quotes).

-f file

Evaluate file.

--apply expr

Apply function to result.

--impure

Allow impure evaluation.

Simple expression

nix eval --expr 'builtins.length [1 2 3]'

Flake attribute

nix eval .#packages.x86_64-linux.hello.version

With function application

nix eval nixpkgs#lib --apply 'lib: lib.version'

$
# CAVEATS

Part of experimental Nix 3.x commands. Flakes must be enabled. Different from nix-instantiate syntax.

# HISTORY

nix eval was introduced as part of the Nix 2.4 flakes experimental features, redesigning the Nix CLI for better ergonomics.

# SEE ALSO

nix(1), nix-instantiate(1), nix-build(1)